One of the most usual enhanced oil recovery methods consists of scavenging the oil field by injection of injection of salty water, thus forcing the oil to sweat and flow out from the rock where it is adsorbed.
The efficiency of such a method is very often limited by the viscosity difference between oil and water, this difference being responsible for the tendency of water, instead of spreading itself over the whole area of the field, to find preferential paths, directly from the injection well to the production well.
In order to compensate for this viscosity difference, it is usual to thicken the injected water by addition of hydrosoluble polymers, very often an acrylamide or a polysaccharide polymer or copolymer.
For this purpose the polymeric material must be dissolved in water. Now, this dissolution is often difficult to achieve in view of the low solubilization rate and of the difficulty to subsequently disperse the solid in water.
On the other hand, accumulations of dispersed grains are immediately sealed in contact with water and form gelatinous conglomerates (micelles-microgels). A strong and extensive stirring provides for the dissolution of a substantial amount of said aggregates but it is practically impossible to stir over a very long period with a sufficient power.
In addition, the storage and handling on the field of these polymers rise many practical problems.
As a matter of fact, the polymer powders have the property to absorb the air moistness. This absorption results thereafter in a swelling and in the formation of conglomerates and/or microgels.
The conglomerates tend to stick to the walls and thus to block the operation of the mixing equipment, particularly the feed screw.
The microgels are not substantially dissolved in water and, once injected in the fields, they tend to generate clogging phenomena by blocking the outlet pore openings of the oil rocks.
On the other hand, the polymer powder, spreading on the earth and in air, rises security problems, making the ground thick and slippery and the atmosphere difficultly breathable.
In order to cope with these various disadvantages, different techniques for dispersing solid particles in an aqueous medium have been proposed in the prior art.
Thus it has been proposed in the prior art, for enhanced oil recovery, to use polymers aqueous emulsions directly prepared during the synthesis of the polymer, as taught, for example, in U.S. Pat. Nos. 3,637,564, 3,734,873 and 3,763,071. The water addition required in this technique is however unfavorable both for the transportation of the product and the storage thereof on the field.
Different techniques for dispersing these particles of hydrosoluble polymers in a liquid which would be non-solvent for the polymer have already been proposed in the prior art. Thus, it has been taught to admix polyacrylamide with glycerine (e.g. in U.S. Pat. No. 3,839,202), with polyethylene glycol (e.g. in U.S. Pat. No. 3,402,137 or British Pat. No. 1,387,367), with ethylene glycol in the presence of an emulsifying agent (e.g. in U.S. Pat. No. 3,657,182) or still with an organic liquid not miscible with water, to which water is added so as to swell the polymer (e.g. in U.S. Pat. No. 3,282,874).
However the main difficulty encountered during the preparation of the polymers suspensions, according to said latter techniques, consists in the instability of the suspensions during their storage, due to their settling or their thickening as the result of a progressive swelling of the polymer in the presence of one of the ingredients; this instability during time results in the formation of compositions which are no longer uniform and which may become more or less compact. Their use is accordingly more difficult and risky.
Hydrosoluble polymers compositions and more particularly compositions of ethylene oxides polymers, which quickly dissolve in water, have been disclosed, for example, in the European patent application No. 0 002 368. When fluid compositions are obtained by dispersing the hydrosoluble polymer in an organic liquid insoluble in water, in the presence of a non-ionic emulsifying agent, a settling of the compositions is observed during their storage at ordinary temperature. It is for this reason that it has also been proposed to add to the compositions a thickening agent such as finely dispersed silica, asbestos or even soaps such as aluminum stearate. However, said compositions are not suitable for the subsequent preparation of polymers aqueous solutions destined to enhanced oil recovery techniques. As a matter of fact, it has been observed that the thickening agents proposed in the prior art produce secondary reactions, either of cross-linking or of degradation, in the presence of hydrosoluble polymers of the type used for enhanced oil recovery, and more particularly of acrylamide polymers or copolymers. These secondary reactions result in the formation of microgels which plug the porous media where they produce a viscosity decrease of the aqueous solutions and, accordingly, a loss in efficiency for the oil recovery.
Another technique, disclosed in the French Pat. No. 2,486,950 consists of producing relatively stable anhydrous suspensions in an aliphatic or aromatic liquid non solvent of the considered hydrosoluble polymers which are kept uniform and fluid during the storage and are quickly redissolved when dispersed in aqueous phase, finally giving diluted aqueous solutions suitable for enhanced oil recovery. However the use of said techninique is not sufficiently satisfactory.